1. Field of the Invention
The present invention relates to an exhaust pipe collecting structure obtained by extending exhaust pipes from exhaust ports of cylinders of a multi-cylinder engine, for example, a four-cylinder engine, and by collecting these exhaust pipes, and a personal watercraft (PWC) which is equipped with the engine having the exhaust pipe collecting structure and is configured to eject water rearward and plane on a water surface as the resulting reaction.
2. Description of the Related Art
A multi-cylinder engine unit, for example, a four-cylinder engine unit, is mounted as a motor in various vehicles including jet-propulsion personal watercraft. In recent years, the jet-propulsion personal watercraft have been widely used in leisure, sport, rescue activities, and the like. The personal watercraft is configured to have a water jet pump that pressurizes and accelerates water sucked from a water intake generally provided on a bottom hull surface and ejects it rearward from an outlet port. As the resulting reaction, a body of the jet-propulsion personal watercraft is propelled. In the jet-propulsion personal watercraft, a steering nozzle provided behind the outlet port of the water jet pump is swung either to the right or to the left by operating a bar-type steering handle to the right or to the left, to change the ejection direction of the water to the right or to the left, thereby turning the watercraft to the right or to the left.
In a multi-cylinder engine unit, such as the four-cylinder engine unit, exhaust pipes extend from exhaust ports of the cylinders and are collected into one exhaust passage, and a downstream end of the exhaust passage is connected to a muffler. An exhaust gas is discharged outside through the muffler. Such a multi-cylinder engine unit typically uses an exhaust pipe collecting structure in which a plurality of exhaust pipes (four or more pipes) are collected into one exhaust passage in the vicinity of the engine. The exhaust pipe collecting structure is generally formed in an exhaust manifold.
When an exhaust pipe collecting structure is accommodated in a relatively wide engine room such as in an automobile, the four exhaust pipes may be formed having a two-part parting plane that is one continuous plane and passes through the location where the exhaust pipes are collected. The continuous plane may be a flat, but is not necessarily so. However, when the exhaust pipe collecting structure is accommodated in a limited space around the engine of the personal watercraft or small all-terrain vehicle, exhaust efficiency and efficient use of the space should be taken into account. This makes it difficult for the parting plane of the exhaust pipes to be located on one continuous plane, passing through the location where the exhaust pipes are collected. Consequently, casting using a mold also becomes difficult.
In addition, in the personal watercraft, extension of the total length of the exhaust passages, which leads to higher performance of the engine unit, is not accomplished because of the limited space. Alternatively, in order to produce the same effects obtained by extension of the exhaust passages by surrounding the exhaust passage with a water jacket, the temperature of the exhaust passages is reduced to cause a sound speed of exhaust gases flowing inside the exhaust passages to be reduced. However, to form the water jacket, it is necessary to form a double-walled structure in the exhaust pipes and the exhaust passages, which requires more complex technology of casting using the mold.
As described above, when the parting plane of the exhaust pipe collecting structure is not located on one continuous plane in casting, casting using the mold becomes difficult. As another alternative solution to this, steel pipes are welded to be formed into the exhaust pipe collecting structure. In this case, because a plurality of parts (e.g., pipe members) having complex shapes are welded, productivity is reduced as compared to the exhaust pipe collecting structure cast using the mold. In addition, in order to manufacture the exhaust pipe collecting structure having the double-walled structure to form the water jacket around the exhaust passage, difficult works need to be conducted.
Further, in the exhaust pipe collecting structure, at a joint portion where the exhaust passages of the exhaust pipes are collected into one exhaust passage or in the vicinity thereof, the shapes of the exhaust passages having an equal total length are varied from one another. In this structure, the variation in the shapes of the arranged exhaust passages makes the entire exhaust pipe collecting structure bulky, which imposes limitation on arrangement of the water jacket.